1. Field of the Invention
The present invention relates to a liquid crystal display device and a manufacturing method thereof.
2. Description of the Related Art
As a display device which is thin and lightweight (a so-called flat panel display), a liquid crystal display device including a liquid crystal element, a light-emitting device including a self-light-emitting element, a field emission display (an FED), and the like have been competitively developed.
As a display method of a liquid crystal display device, a twisted nematic (TN) mode is typically employed. However, a TN-mode liquid crystal display device, in which an electric field is applied in a direction perpendicular to a liquid crystal layer, has a disadvantage of a large change in color or luminance which depends on viewing angle, that is, narrowness of the viewing angle in which normal display can be recognized.
By contrast, a horizontal electric field mode such as an in-plane-switching (IPS) mode can be given as a display method of a liquid crystal display device which is often used besides the TN mode. In the horizontal electric field mode, liquid crystal molecules are driven by applying an electric field in a direction parallel to a substrate, unlike in the TN mode. Accordingly, a horizontal electric field mode liquid crystal display device can have a wider viewing angle than a TN-mode liquid crystal display device. However, a horizontal electric field mode still has problems in contrast ratio, response time, and the like.
A ferroelectric liquid crystal (FLC) mode, an optical compensated birefringence (OCB) mode, and a mode using a liquid crystal exhibiting a blue phase can be given as display modes with shortened response time of liquid crystal molecules.
In particular, a display mode using a liquid crystal exhibiting a blue phase enables high-speed response because the liquid crystal exhibiting a blue phase has a short response time of 1 msec or less, and in addition has a variety of advantages such as nonnecessity of an alignment film and a wide viewing angle (for example, see Patent Document 1).
In recent years, liquid crystal display devices having larger size and higher definition have been developed. When high-speed response is achieved, moving image display performance is improved and high-quality moving image can be provided. However, for larger size and higher definition of liquid crystal display devices, a decrease in driving speed of the liquid crystal display devices needs to be suppressed by reducing parasitic capacitance and resistance of wirings to display a high-quality moving image.
As the size and the definition of liquid crystal display devices increase, the number of pixels is increased, and the number of portions where scan lines and signal lines intersect with each other is increased accordingly. Thus, total parasitic capacitance between the scan lines and the signal lines is increased, and a problem of signal delay may be caused. In the case where the number of pixels in a liquid crystal display device is small, the above-described problem of signal delay is negligible, while in the case where the size and the definition of a liquid crystal display device are increased and the number of pixels is increased, the above-mentioned parasitic capacitance prevents an increase in driving speed of the liquid crystal display device.